detect arm64-linux platform;

--- a/lib/scripts/isabelle-platform	Tue Sep 08 11:32:57 2020 +0200
+++ b/lib/scripts/isabelle-platform	Wed Sep 30 13:20:33 2020 +0000
@@ -12,8 +12,15 @@
 case $(uname -s) in
   Linux)
     ISABELLE_PLATFORM_FAMILY="linux"
-    ISABELLE_PLATFORM32=x86-linux
-    ISABELLE_PLATFORM64=x86_64-linux
+    case $(uname -m) in
+      aarch64)
+        ISABELLE_PLATFORM64=arm64-linux
+        ;;
+      *)
+        ISABELLE_PLATFORM32=x86-linux
+        ISABELLE_PLATFORM64=x86_64-linux
+        ;;
+    esac
     ;;
   Darwin)
     ISABELLE_PLATFORM_FAMILY="macos"

--- a/src/Pure/System/platform.scala	Tue Sep 08 11:32:57 2020 +0200
+++ b/src/Pure/System/platform.scala	Wed Sep 30 13:20:33 2020 +0000
@@ -38,11 +38,13 @@
 
   private val X86 = """i.86|x86""".r
   private val X86_64 = """amd64|x86_64""".r
+  private val Arm64 = """arm64|aarch64""".r
 
   def cpu_arch: String =
     System.getProperty("os.arch", "") match {
       case X86() => "x86"
       case X86_64() => "x86_64"
+      case Arm64() => "arm64"
       case _ => error("Failed to determine CPU architecture")
     }